1. Field of the Invention
The present invention relates to a process for forming a fine pattern employed in pattern formation by a charged beam (e.g. electron beam, focus ion beam) direct writing to obtain a semiconductor device or an integrated circuit.
2. Description of the Prior Art
In production of electronic devices such as IC, LSI, and the like, pattern formation has hitherto been conducted by photolithography using ultraviolet rays. As the pattern dimensions of these devices have become finer, it is suggested to use a stepper lens of higher numerical aperture (NA), a light source of shorter wavelength, etc., but it invites a drawback of smaller focus depth. Further, electron beam lithography has come to be used as the pattern dimension of LSI devices has become finer and the production of ASIC has started.
In the fine pattern formation by electron beam lithography, a positive type electron beam resist is requisite. A polymethyl methacrylate (PMMA) is known as a positive type electron beam giving the highest resolution, but has a drawback of low sensitivity.
Therefore, there have been presented, in recent years, many reports concerning the enhancement of sensitivity of positive type electron beam resists. These reports propose positive type electron beam resists of, for example, polybutyl methacrylate, a copolymer of methyl methacrylate and methacrylic acid, a copolymer of methacrylic acid and acrylonitrile, a copolymer of methyl methacrylate and isobutylene, polybutene-1-sulfone, poly(isopropenyl ketone) and fluoro polymethacrylate.
In all of these resists, in order to obtain a high sensitivity, an electron withdrawing group has been introduced into the side chain or an easily decomposable bond has been introduced into the principal chain to allow the principal chain to undergo easy scission by electron beam. However, they do not fully satisfy both of resolution and sensitivity. Further, they are not sufficiently good in dry etch resistance and heat resistance. Consequently, it is difficult to use them as a mask for dry etching and their usages are limited.
Meanwhile, negative type electron beam resists using a cyclized rubber as a base have drawbacks in that they have low adhesion to a substrate, are difficult to form a uniform high-quality film with no pinholes on a substrate, and have low thermal stability and resolution. Therefore, various improvements have hitherto been made for negative type electron beam resists. As a result, there have been proposed negative type electron beam resists such as poly(glycidyl methacrylate), chloromethylated polystyrene, chloromethylated .alpha.-methyl polystyrene, polymethacrylate maleic acid ester, chlorinated polystyrene, glycidyl methacrylate-ethyl acrylate copolymer and the like. In all of these resists, epoxy groups or chlorine atoms, sensitive to electrons, have been introduced in order to allow them to generate radicals easily when irradiated with an electron beam and give rise to a crosslinking reaction. The above resists are intended to have a high sensitivity but are not sufficient in any of resolution and heat resistance.
In developing a negative type resist using a rubbery thermoplastic polymer (e.g. the above cyclized rubber or a polyisoprene) as a base, an organic solvent is required as a developer. In some cases, this organic solvent developer causes swelling of an image-written resist during the development of the resist, which reduces the resolution of pattern and, in some cases, invites distortion of pattern and makes the pattern unusable. Moreover, the organic solvent developer is harmful from the standpoints of environment and human health and additionally has flammability.
Electron beam lithography has various drawbacks such as poor dry etch resistance and heat resistance of an electron beam resist, adverse effect of proximate effect caused by forward or backward scattering of electron on pattern precision, adverse effect of charging of incident electrons on pattern writing, and the like.
In order to improve these drawbacks, use of a multi-layer resist consisting of a pattern forming layer and a planarizing layer is very effective. FIGS. 4A to 4D are illustrations explaining a process for forming a tri-layer resist by electron beam lithography. In order to expect a reduced proximate effect, a high-molecular organic film as a bottom layer 31 is formed on a substrate 11 in a thickness of 2-3 .mu.m and then a heat treatment is effected (FIG. 4A). Thereon is formed, as an intermediate layer 32, an inorganic film of SiO.sub.2 or the like or an inorganic high-molecular film of SOG (spin on glass) or the like in a thickness of 0.2 .mu.m. Thereon is formed, as a top layer 33, an electron beam resist in a thickness of 0.5 .mu.m. Thereon is vapor-deposited a thin aluminum film 34 in a thickness of about 100 .ANG. in order to prevent the charging (FIG. 4B). Then, writing by an electron beam 35 is effected; the thin aluminum film is removed with an aqueous alkali solution; and development is effected to obtain a resist pattern 33P (FIG. 4C). Thereafter, the intermediate layer is dry-etched using the resist pattern as a mask, after which the bottom layer is dry-etched using the intermediate layer as a mask (FIG. 4D). By employing the above process using a multi-layer resist, a fine pattern can be formed at a high aspect ratio. However, in the process using a multi-layer resist on which a thin aluminum film is vapor-deposited, the steps are more complex; contamination is higher; dimensional change in pattern transfer is larger; thus, the process using the multi-layer resist is not practical.
As shown in FIG. 5, in the conventional process using a tri-layer resist, it occurs in some cases that a width of 0.5 .mu.m in design pattern 40 becomes thinner to about 0.3 .mu.m.
As mentioned above, the process using a multi-layer resist having a thin aluminum film thereon is effective, but has various drawbacks such as complex steps, aluminum contamination, change in resist dimension during pattern transfer, and the like.
In the process using a multi-layer resist having no thin aluminum film, there is a problem of charging. This charging is a phenomenon in which incident electrons are accumulated in the resist, the intermediate layer or the bottom layer, all being an insulator. The charging invites serious problems in electron beam lithography, for example, reduction in field butting accuracy and overlay accuracy. The charging is also seen in a single-layer resist and, as in the tri-layer resist, invites reduction in field butting accuracy and overlay accuracy. That is, in electron beam lithography, incident electrons are scattered in the resist but stop at a depth of 1-1.5 .mu.m from the resist surface, whereby the incident electrons are accumulated in the resist pattern at the depth. It is thought that these accumulated electrons cause deflection of electron beam, which in turn invites reduction in field butting accuracy and overlay accuracy.
FIG. 6 is a drawing showing the surface of a pattern formed in a process using a tri-layer resist having no aluminum film thereon, prepared based on a scanning electron micrograph of said surface. It shows that charging has caused field butting error and pattern breakage. That is, when electron beam exposure is effected by scanning an electron beam over a region A as shown in an arrow O and then over a region B as shown in an arrow P to connect the regions A and B, the presence of charging (accumulated electrons) causes breakage (field butting error) 100 between the region A and the region B in the resist pattern 50 after development.
The present inventors completed an electron beam resist having high sensitivity and conductivity, capable of solving the above problems, as well as a process for forming a fine pattern using the resist.